Liquid Crystal Displays (LCDs) are used in a variety of products, including hand-held games, hand-held computers, and laptop/notebook computers. These displays are available in both gray-scale (monochrome) and color forms, and are typically arranged as a matrix of intersecting rows and columns. The intersection of each row and column forms a pixel, or dot, the density and/or color of which can be varied in accordance with the voltage applied to the pixel in order to define the gray shades of the liquid crystal display. These various voltages produce the different shades of color on the display, and are normally referred to as “shades of gray” even when speaking of a color display.
The image displayed on the screen may be controlled by individually selecting one row of the display at a time, and applying control voltages to each column of the selected row. The period during which each such row is selected may be referred to as a “row drive period”. This process is carried out for each individual row of the screen; for example, if there are 480 rows in the array, then there are typically 480 row drive periods in one display cycle. After the completion of one display cycle during which each row in the array has been selected, a new display cycle begins, and the process is repeated to refresh and/or update the displayed image. Each pixel of the display is periodically refreshed or updated many times each second, both to refresh the voltage stored at the pixel as well as to reflect any changes in the shade to be displayed by such pixel over time.
LCDs used in computer screens require a relatively large number of such column driver outputs. Color displays typically require three times as many column drivers as conventional “monochrome” LCD displays; such color displays usually require three columns per pixel, one for each of the three primary colors to be displayed.
The column driver circuitry is typically formed upon monolithic integrated circuits. Integrated circuits which serve as column drivers for active matrix LCD displays generate different output voltages to define the various “gray shades” on a liquid crystal display. These varying analog output voltages vary the shade of the color that is displayed at a particular point, or pixel, on the display. The column driver integrated circuit must drive the analog voltages onto the columns of the display matrix in the correct timing sequence.
LCDs are able to display images because the optical transmission characteristics of liquid crystal material change in accordance with the magnitude of the applied voltage. However, the application of a steady DC voltage to a liquid crystal will, over time, permanently change and degrade its physical properties. For this reason, it is common to drive LCDs using drive techniques which charge each liquid crystal with voltages of alternating polarities relative to a common midpoint voltage value. It should be noted that, in this context, the “voltages of alternating polarities” does not necessarily require the use of driving voltages that are greater than, and less than, ground potential, but simply voltages which are above and below a predetermined median display bias voltage. The application of alternating polarity voltages to the pixels of the display is generally known as inversion.
Accordingly, driving a pixel of liquid crystal material to a particular gray shade involves two voltage pulses of equal magnitude but opposite polarity relative to the median display bias voltage. The driving voltage applied to any given pixel during its row drive period of one display cycle is typically reversed in polarity during its row drive period on the next succeeding display cycle. The pixel responds to the RMS value of the voltage so the final “brightness” of the pixel only depends on the magnitude of the voltage and not the polarity. The alternating polarity is used to prevent “polarization” of the LC material due to impurities.